1. Technical Field
The present invention relates to a new copolymer effectively used as a binder resin of an aqueous ink and a pigment-dispersed resin, an aqueous ink including the copolymer, and an ink cartridge.
2. Description of the Related Art
Inkjet recording methods are advantageous in comparison with other recording methods in that since the process is simple, full colorization is easy and high definition images can be obtained by a device having a simple configuration. For this reason, the inkjet recording is widely diffusing from home use to office use, commercial printing, and industrial printing. In such an inkjet recording, aqueous ink compositions using hydrosoluble dye as coloring materials are mainly used. However, its water resistance and light resistance are inferior so that the development of a pigment ink using a water insoluble pigment is in advance in place of the hydrosoluble dye.
For inkjet ink printing for office use, recording media, typically plain paper, are used and high image density is demanded. In general, when images are printed on plain paper using a pigment ink, the pigment ink does not stay on the surface of the paper but permeates into the paper, so that the pigment density on the surface decreases and consequently the image density lowers. As the pigment concentration in the ink increases, the image density increases. However, the ink becomes viscous, thereby degrading the discharging stability of the ink.
Moreover, water contained in the pigment ink swells the surface of plain paper immediately after the ink droplets land on the paper. As a result, the extension percentage differs between the top surface and the bottom surface of the paper, thereby causing the paper to curl. This kind of phenomenon does not create a problem at low speed printing since curling is canceled as drying proceeds. However, as the printing speed increases, recording media are transferred before curling is canceled after printing, which naturally leads to occurrence of paper jam. To prevent paper jam, it is suitable to add a permeating agent to a pigment ink to promote water to permeate into paper. However, ink becomes hydrophobic by the agent, which makes it difficult to secure storage stability of the ink. Also, the pigment ink more easily permeates into a recording medium, thereby furthermore decreasing the image density.